The long-term goal of this program project is to understand the molecular mechanisms that allow Arp2/3 complex to control actin filament assembly during endocytosis and cellular motility. The mechanism of action of Arp2/3 complex is so complicated that a team effort is required. Our group of four geographically dispersed laboratories combines x-ray crystallography, electron microscopy, image processing and molecular biology to address technically challenging questions. Structural insights from his ongoing collaboration will provide important clues about reaction mechanisms and also help to design experiments to measure kinetic and thermodynamic constants and to study actin assembly in live cells. Project 1 (Pollard) proposes a combination of biochemical and x-ray crystallographic experiments to obtain detailed structural information regarding conformational changes and macromolecular interactions of Arp2/3 complex. Project 2 (Li) proposes a combination of biophysical studies on interaction's of nucleation promoting factors with purified Arp2/3 complex and microscopic analysis of mouse cells lacking subunits of Arp2/3 complex. Project 3 (Hanein) proposes electron microscopy of purified Arp2/3 complex, actin filament branches and of actin networks in mouse cells with and without functional Arp2/3 complex. Project 4 (Volkmann) proposes computational methods to extract the maximal amount of information from spectroscopy experiments in projects 1 and 2 and the electron micrographs from project 3.